Modern aircraft wings often include a series of movable flight control surfaces, known as flaps or slats, that can be selectively extended or retracted to modify the lift producing characteristics of the wings. Extension and retraction of such flaps or slats is accomplished by a flight control actuation system mounted in the wing.
A typical actuation system includes a series of actuators spaced along the span of each wing, and operably connected to move one or more individual flight control surfaces. Adjacent actuators are connected to each other by drive shafts, to in essence form a chain of actuators and shafts extending along the span of the wing. A power drive unit (PDU) connected to the inboard end of the chain provides motive power for driving the actuators to selectively extend or retract the flight control surfaces.
Because control surfaces such as flaps or slats significantly alter the lift producing characteristics of the wings, it is critical for safe operation of the aircraft that the actuation system also include safety features for detecting and reacting to problems such as jamming, or failure of one of the actuators or drive shafts in the aircraft flight control system. Of particular concern are problems which cause the position of the flaps or slats on one wing to lose synchronization with the flaps and slats on the other wing of the aircraft. Such a condition is referred to as asymmetry. To prevent asymmetry, actuation systems for flaps and slats often include a device known as an asymmetry brake which engages to hold the chain of actuators and shafts in a known position, should a problem occur in the actuation system that cannot be corrected through use of the power drive unit alone. For example, should one of the shafts connecting adjacent actuators break, the PDU would not be able to control the position of flaps or slats outboard of the broken shaft. Without some means, such as an asymmetry brake at the outboard end of the chain of actuators and shafts, for holding the flaps or slats downstream from the broken shaft against further movement, aerodynamic loads acting upon the flaps or slats could move them to an uncommanded position which would create serious flight control problems for the aircraft.
U.S. Pat. No. 3,662,550 to Lichtfuss, U.S. Pat. No. 4,779,822 to Burandt et al., and U.S. Pat. No. 5,484,043 to Quick et al., describe flight control actuation systems and asymmetry brake devices such as those described above. As will be readily apparent from these patents, actuation systems for critical aircraft flight control surfaces, such as flaps and slats, are designed to have a high degree of redundancy for monitoring and reacting to problems which could lead to asymmetry.
On one recently designed aircraft, very narrow, supercritical, wings were utilized to minimize fuel consumption. The wings were so narrow at their tips, that there was not enough space within the wing for mounting an asymmetry brake at the outboard end of the chain or actuators and interconnecting shafts, as in prior flight control systems.
As a result, a novel approach was developed in which the asymmetry brake was positioned between the two outermost actuators, and the outermost actuator was provided with an integral no-back device to maintain position of the outermost actuator, in the event that the driving connection fail between the asymmetry brake and the outermost actuator. This actuation system is described in detail in co-pending patent application Ser. No. 08/602,190, which is assigned to the assignee of the present invention and incorporated herein by reference.
Even with the asymmetry brake repositioned between the two outermost actuators, however, there was still insufficient space within the wing to house an asymmetry brake of any known prior construction at the new location. It was, therefore, necessary to develop a new, more compact asymmetry brake.
In addition to making the brake physically smaller, a number of other design constraints made designing a new asymmetry brake for the new aircraft described above a significant challenge. The overall design of the flight control system required that the brake be applied and released several times during each flight of the aircraft. This requirement ruled out the use of many prior brake designs which could only be reset manually on the ground once they had been triggered in flight.
The actuators used on the new aircraft were of a type having little inherent friction and are thus readily backdrivable by aerodynamic loads. This created high backdriving loads which had to be reacted by the brake. Braking devices using friction plates, of the type utilized in some prior asymmetry brakes, having enough braking capacity to react the backdriving loads were physically too large to fit within the available space.
The large backdriving loads created an additional problem in that the actuator for engaging and disengaging the brake had to be capable of overcoming the large backdriving loads to apply or release the brake. The overall design of the flight control system required that the brake actuator be electrically operated. The small available space did not allow for the use of an electrical motor and geartrain. Existing electrical solenoid designs were physically too large, or required too much current. To make matters worse yet, the overall system design required that the electrical actuation means utilized in the brake incorporate redundant features which would allow the actuator to apply full rated engagement force when supplied with current from either of two sources of electrical current which were electrically isolated from one another. This meant that the solenoid had to have two separate windings, each capable of generating full rated force of the solenoid.
In its new location, the asymmetry brake had to include a thru-shaft, making attempts to diametrically shrink the size of the brake more difficult.
In addition to providing a design meeting the requirements listed above, it was also desired to provide an improved asymmetry brake which would significantly reduce the incidence of nuisance trips and irregular operation experienced with prior asymmetry brakes.
Accordingly, it is an object of our invention to provide an improved braking apparatus, suitable for use in an application such as the flight control actuation system for the new aircraft described above. It is also an object of our invention to provide such a braking apparatus that is physically compact, powerful, and highly reliable in a form that includes a minimal number of parts of straightforward design which can be produced at reasonable cost.